Light directed into a turbid medium undergoes two phenomena; scattering and absorption. In tissue, for example, the amount of scattering is determined by the tissue structure such as cell and mitochondria size, while the absorption is determined by the quantity of endogenous absorbers such as melanin and porphyrins (e.g. hemoglobin in blood). Different tissue types scatter and absorb light in different amounts, i.e. liver versus muscle. Many recent pharmacokinetic studies are using fluorescent drugs to monitor body processes. Also, new forms of cancer treatments use fluorescent drugs. These treatment methods require accurate knowledge of the drug concentration for proper treatment.
A possible method of determining the concentration of the fluorescent drug is to measure its fluorescence. The strength of the fluorescence signal however, will depend on the intensity of excitation light, and the scattering and absorption properties of the turbid medium.
U.S. Pat. No. 4,178,917 to Shapiro discloses a method and system for non-invasive detection of zinc protoporphyrins using an excitation beam and two detectors, one being termed the reference detector for measuring scattered or reflected light S.sub.2 at the excitation wavelength. The fluorescence detector measures the fluorescence S.sub.1 from the red blood cells flowing through the measurement volume. The intent of this device, therefore, is to measure only the concentration of fluorophores within the blood stream and not in the surrounding tissue. As a result, the signal is influenced by the intervening tissues and the method described does not correct for differences in these tissues. A further drawback to this method is that it is not readily adaptable for measuring fluorophore concentrations in a wide range of turbid media.
It would therefore be very advantageous to provide a method for measuring in vivo concentration of a luminescent material in a turbid medium substantially independent of the scattering and absorption properties of the medium.